When fabricating integrated circuits, various features such as metal lines are formed into and onto a semiconductor substrate. To form these features, photo-masks are used to form a pattern into a photo-resist layer. The regions where the photo-resist layer is removed expose the underlying substrate to an etching process used to form trenches where metal is subsequently placed.
As the patterns formed into photo-resist layers become increasingly dense, it becomes difficult to use a single photo-mask to form a pattern in the photo-resist layer because features within the nanometer range are smaller than the resolution of a light source to which the photo-resist layer is exposed. Thus, multiple masks may be used to form the features within a pattern. Specifically, each of the multiple masks is used to create different features within the final pattern.
When multiple masks are used to form a single pattern, it is important that the patterns in the different masks are aligned properly so that various errors may be avoided. For example, it is problematic if two adjacent features, each from a different mask, are formed too close to each other. Such errors may cause short circuits or bad circuit connections.
In some cases, a cut pattern is used in association with a main pattern to form a target pattern. The cut pattern removes features formed by the main pattern in order to achieve the desired target pattern. Using such a technique provides certain advantages to the photolithographic process. For example, it is desirable to have a large process window. The process window refers to a range of focus and exposure settings that will still produce the desired features into the photo-resist layer. A process window can be improved by keeping features within a pattern relatively uniform in density. This may involve placement of “dummy” features in or near the pattern. Dummy features are extra features that are placed in order to maintain feature density, but may not serve any function within the circuit for which the pattern is designed. To separate dummy features from real features and to create the desired pattern, a cut mask can therefore be used.
When a cut mask is used, an overlay budget and CD variation budget of the cut mask are important considerations. In some occasions, only one feature at a minimum pitch will be cut. Therefore, a very small cut feature is needed. The pattern fidelity of such a small feature is often not desirable, and the total budget for overlay and CD variation is insufficient.
Accordingly, what is needed is a method to address both the overlay error impact in multiple patterning and the overlay budget and CD variation budget of cut patterns.